Transformer oil



Oct. 15, 1968 R. WYNKOOP ET AL 3,406,111

TRANSFORMER OIL Filed July 12, 1962 5 Sheets-Sheet l Oxidation Time,Hrs.

INVENTO RAYMOND WYNKO I SHIRLEY C. BARTLETT,JR.

V XM

ATTORNEY Oct. 15, 1968 WYNKQOP ET AL 3,406,111

TRANSFORMER OIL 5 Sheets-Sheet 2 Filed July 12, 1962 QEP Q 3,9 10;:03JaMo wag ma I I RAYMOND WY NVENTORJ NKOOP SHIRLEY C. BARTLETT, JR

ATTORNEY Oct. 15, 1968 WYNKOQP ET AL 3,406,111

TRANSFORMER OIL Filed July 12, 1962 '5 Sheets-Sheet 5 4'0 Ibo GzsidaiionTime, Hrs.

INVENTORS RAYMOND WYNKOOP SHIRLEY C. BARTLETT, JR.

ATTORNEY United States Patent 3,406,111 TRANSFORMER OIL Raymond Wyukoop,Gladwyne, and Shirley C. Bartlett, IL, Broomall, Pa, assignors to SunOil Company, Philadelphia, Pa., a corporation of New JerseyContinuation-impart of application Ser. No. 94,433, Mar. 9, 1961. Thisapplication July 12, 1962, Ser. No. 209,345

4 Claims. (Cl. 208-14) This invention relates to novel transformer oilcompositions and more particularly to uninhibited transformer oilswhich, under Doble Oxidation Test conditions, consistently exhibit powerfactors less than two percent throughout surprisingly long periods ofoxidation and also low sludging tendency and high oxidation stabilityunder the test conditions.

This application is a continuation-in-part of copending application Ser.No. 94,433, filed Mar. 9, 1961, now abandoned, which in turn is acontinuation-in-part of application Ser. No. 15,810, filed Mar. 18, 1960and now abandoned.

Properties of commercial oils used as insulating media in transformersare well known in the art and a list of typical characteristics is givenin the text by F. M. Clark entitled, Insulating Materials for Design andEngineering Practice (1962), page 135. Such oils typically boil in therange of 460-775 F. and preferably have viscosities in the range of55-60 S.U.S. at 100 F., as may be seen by reference to Wasson et al.Patent No. 3,000,807.

Commercial transformer oils customarily are tested by the DobleOxidation Test for electrical insulating oils developed by the DobleEngineering Company of Belmont, Massachusetts. This procedure has beendescribed in ASTM Standards on Electrical Insulating Liquids and Gases,pp. 307-313, December 1959, under the title Suggested Method of Test forOxidation Characteristics of Mineral Transformer Oil. It involvesbubbling air through a known amount of the oil held at a temperature of95 C. in the presence of copper and iron and making two types of testsdaily on small samples of the oil. One type of test is an aciditymeasurement. The other is a precipitation test in which one volume ofthe oil is diluted with five volumes of pentane, the mixture is allowedto stand at least eight hours and the presence or absence of a sludgeprecipitate is noted. The endpoint of the Doble test is taken as thenumber of days of oxidation either before the acidity of the oil reaches0.25 mg. of KOH per gram or before a positive precipitation test forsludge is obtained. The best commercial transformer oils heretoforeavailable generally have a life of only about three days under Dobletest conditions. Longer life values could be obtained by adding anoxidation inhibitor to the oil but the use of such additivetraditionally has been considered unacceptable by transformermanufacturers and users.

An amplification of the Doble test recommended by the Doble EngineeringCompany is the so-called Power Factor Valued Oxidation (PFVO). Thisinvolves operating in the manner described above but also determiningthe power factor of the oil at two hour intervals throughout theoxidation period. A curve is obtained by plotting the power factoragainst the oxidation time. The accompanying FIGURE 1 illustrates thetype of curves found to be characteristic of commercially availabletransformer oils. FIGURE 1 shows two curves that were obtained forcommercial transformer oils A and B. It can be seen that for each oil asharp hump in the power factor curve occurs in the earlier stages ofoxidation. The power factor rapidly increases to a value considerably inexcess of two percent, following which it decreases to a relatively lowvalue and thereafter rises continuously. The time of oxiice dation atwhich the peak of the hump is reached may differ with different oils,but it is characteristic for all uninhibited commercial transformer oilspreviously available that a hump in the PFVO curve is obtained and thatthe power factor at the peak of the hump is considerably above twopercent. The ideal transformer oil of course would be one that exhibitsno increase whatever in power factor throughout the test life.

Transformer oils of the present invention are unlike any previouslyavailable in that the power factor curve obtained is relatively fiat.This is illustrated by FIG- URES 2-A and 2-B representing threeuninhibited transformer oils (C, D and E) according to the presentinvention. These oils were prepared by the process described and claimedin the aforesaid copending applica tion Ser. No. 94,433, hereinafterdescribed. FIGURE 2-A shows the power factor curves for the three oilsduring the first 200 hours of oxidation by the Doble procedure, whileFIGURE 2-B presents an extension of the curves for the oxidation periodof 200-400 hours. It can be seen that the curves are remarkably fiat ascompared to those obtained for other transformer oils. While there maybe a slight hump in the power factor curve for some of the oils of thepresent invention, the hump is insignificant and the power factorconsistently remains less than two percent for at least 96 hours ofoxidation (4 days). Preferred compositions, such as the oils illustratedin FIGURES 2-A and 2-B, have power factors that remain consistentlybelow two percent for oxidation times exceeding 192 hours (8 days) andoften considerably in excess of such time.

Transformer oils of the present invention can be characterized asfollows:

(1) They are napthenic petroleum distillates having a viscosity in therange of -65 S.U.S. at 100 F. and more preferably -60 S.U.S. at F.

(2) They have viscosity-gravity constants in the range of 0.84-0.92.This corresponds to aromatic contents generally in the range of 15-65%by weight.

(3) They have nitrogen contents less than 4 ppm. and more preferably notexceeding 2 ppm.

(4) When tested by the Doble Oxidation Test in the absence of aninhibitor, they exhibit the following characteristics: (a) aneutralization number less than 0.25 mg. KOH/ g. at 96 hours oxidationtime; (b) absence of sludge at 96 hours oxidation time; (c) powerfactors during the oxidation period of 0-96 hours consistently less than2 percent. Preferred compositions have power factors that areconsistently below 2 percent for at least 192 hours of oxidation timeand a Doble test life of at least 192 hours (8 days).

The improved transformer oils of the invention can be prepared by theprocess described in the aforesaid application Ser. No. 94,433. Thepreferred procedure involves first treating a naphthenic distillate ofthe abovespecified viscosity range with strong sulfuric acid, removingthe acid sludge and then contacting the oil with adsorptive clay.Typically the oil is contacted with 10 lbs./bbl. of concentratedsulfuric acid and 20 lbs./bbl. of clay. At this stage the oil generallywill have a Doble test life of only about 2 days and a nitrogen contentof the order of 10 ppm. The oil is next treated at 250- 280 F. with analkali metal alkoxide in which the alkoxide portion corresponds to asecondary or tertiary alcohol of the C -C range. The preferred alkoxideis sodium isopropoxide. Other alkali metals, such as potassium orlithium, can be used and the alkoxide portion of the treating agent cancorrespond to other secondary or tertiary alcohols, such as t-butylalcohol, but not to primary alcohols such as ethanol or n-propanol. Thealkoxide can be pre-formed by reacting sodium with an appropriatealcohol or it can be formed in situ in the oil by dispersing metallicsodium therein and then adding the alcohol. Generally it is preferred touse a slight molar excess of alcohol over the stoichiometric proportionrequired to form the alkoxide. The amount of sodium used typically is05-10% by weight on the oil. After treatment with the alkoxide, the oilis contacted at 250-280 F. with carbon dioxide in molar excess of theamount of alkali metal used. Alternatively the oil can be contacted withcarbon dioxide while being treated with the alkali metal alkoxidealthough this is not preferred. The oil is then washed with water toremove the organo-metallic materials and thereafter is heated undervacuum while being sparged with an inert gas (e.g. nitrogen) to removeresidual water. Finally the oil is contacted again at say 225 F. withadsorptive clay in amount, for example, of 20 lbs./bb1. The resultingrefined product will have a nitrogen content less than 4 p.p.m. andgenerally not exceeding 2. p.p.m. and will exhibit the unique Doble testcharacteristics specified above. The product will give outstandingperformance in transformer service.

The foregoing treatment removes little of the aromatic hydrocarbonspresent in the charge stock. The presence of these naturally occurringaromatics in the refined product is highly important in providing theperformance characteristics desired. It appears that the aromaticsfunction as natural oxidation inhibitors and are essential for securingthe unique Doble test characteristics as described above and illustratedin FIGURES 2-A and 2-B. The extremely low nitrogen content obtained bythe above-specitied treatment also is important in contributing to theoutstanding stability of the present oils under the oxidizing conditionsof the Doble test, as it has been found that if the nitrogen contentexceeds 4 p.p.m. the oil generally will have a Doble test life of onlyabout 2 days and a pronounced hump in the initial portion of the powerfactor curve will appear.

Oils which have viscosities substantially dilferent from the range of50-65 S.U.S. at 100 F. curiously do not exhibit the outstanding Dobletest characteristics found in the present oils, even when such otheroils have been treated by the same refining procedure as describedabove. This is illustrated by the PFVO curves shown in FIG- URE 3 fortwo such other oils designated as F and G. Both of these oils werenaphthenic distillates which had been treated by the procedure describedabove using 0.75% sodium by weight on the oil and a molar ratio ofisopropanol to sodium of 1.10. Oil F had a viscosity of 165 S.U.S. at100 F. and a viscosity-gravity constant of 0.882, while thecorresponding properties for oil G were 735 S.U.S. at 100 F. and 0.870.It can be seen from FIGURE 3 that while neither curve exhibits adistinct hump as obtained for the conventional transformer oils ofFIGURE 1, the power factors increased relatively rapidly and exceededtwo percent after only about 40 hours oxidation time. The reason for thepoorer oxidation stabilities of these oils, as compared to oils such asC, D and E of FIGURES 2-A and 2-B, is not known with certainty but it isthought to be likely that the difference in the molecular weight rangesof the aromatics therein is probably responsible. In other words thearomatics in naphthenic oils having viscosities in the range of 50-65S.U.S. at 100 F. appear to have molecular weights and structures suchthat they function more effectively as inhibitors under Doble testconditions than do the aromatics in oils of a substantially differentviscosity range. In any event the fact is that the oil should have aviscosity of 50-65 S.U.S. at 100 F. to exhibit the outstanding Dobletest characteristics illustrated in FIGURES 2-A and 2-B.

The following examples illustrate the preparation of the noveltransformer oils of the invention and properties of the oils soproduced.

EXAMPLE I The charge oil was a naphthenic distillate stock of boilingrange suitable for electrical transformer use. The oil initially had thefollowing properties: A.P.I. gravity: 24.4; flash point:280 F.; firepoint=3l0 F.; S.U.S. viscosity at F.=55.3; S.U.S. viscosity at 210 F:33.6; viscosity-gravity constant=0. 855; nitrogen content=50 p.p.m.;sulfur content=0.18%; refractive index=1.5009; Doble life of a samplewhich was furfural extracted and then treated with 20 lbs./bbl. of 99%acid and 35 lbs./bb1. of clay=2 days. The foregoing Doble test lifeindicates that conventional treatment of this oil does not produce asatisfactory transformer oil.

The charge oil was first contacted at room temperature with 99% sulfuricacid in amount of 10 lbs./bbl. and was water washed and dried by airblowing. The oil was then heated to a temperature of about 270 F., andsodium in amount of 1% by weight was dispersed therein by means of ahigh speed stirrer. Then isopropanol was added in amount of 0.5% byvolume on the oil, whereupon reaction occurred and the temperature roseabout 10 F. over a'time of about 6 minutes and thereafter dropped. Afterabout 30 minutes of mixing, carbon dioxide was bubbled into the mixtureat a temperature of about 245 F. This caused the temperature to riseduring a period of about 10 minutes, the amount of rise being somewhatgreater than during the treatment with sodium and alcohol. Contact withcarbon dioxide was continued for a time of about 10 minutes. Stirring ofthe mixture was then discontinued and it was allowed to stratify whilecooling. Unreacted sodium and a reaction product layer settled to thebottom and the clear oil was decanted. The oil finally was contactedwith 20 lbs./bbl. of clay. Testing of the finished oil showed that ithad a Doble life of 8 days, thus indicating that it would be excellentfor transformer use. The oil product had a nitrogen content of onlyabout 2 p.p.m., which shows that the present treatment is highlyeffective in removing nitrogen compounds. The nitrogenous bodies areconcentrated in the organo-sodium reaction product layer which separatesfrom the oil.

EXAMPLE II Another run was carried out as in the preceding exampleexcept that the amount of sodium used was increased to 2.5% by weightand the isopropanol to 1.0% by volume on the oil. The finished oil inthis case had a Doble life of 11 days,

EXAMPLE III Sodium recovered from the run of Example II was used fortreating another batch of oil in the same manner except that in thiscase contact of the oil with sodium and with carbon dioxide was doneconcurrently. Again a Doble life of 11 days was obtained. This showsthat the excess sodium can be reused and that successive and concurrenttreaments with sodium and carbon dioxide each are efiective in preparingtransformer oils according to the invention.

EXAMPLE IV Another run was carried out in the same manner as in ExampleII except that after the sodium treatment the oil was cooled to roomtemperature before being contacted with carbon dioxide. The resultingoil had a Doble life of 9 days. This shows that an elevated temperatureis not required for the carbon dioxide treatment to be effective.

EXAMPLE V upper layer of clear oil. Dissipation factor tests on the oillayers gave the following results:

Amount of acid used, Dissipation factor lbs./bbl.: at 100 C.

The low dissipation values obtained indicate that most of the sodiumcompounds can be removed from the reaction mixture by centrifuging andthat a final water-washing or clay-treating step is not necessarilyessential for producing a good transformer oil.

EXAMPLE VI The present example illustrates preparation of the presenttransformer oil composition in continuous manner. The reactor used wasprovided with a high speed stirrer and with means for separatelyintroducing streams of the charge oil, the alkoxide reagent which hadbeen pre-formed and carbon dioxide and for continuously withdrawing thereaction mixture. The alkoxide reagent was prepared by forming aconcentrated dispersion of sodium in a small amount of the charge oilheated to a temperature above the melting point of sodium, cooling thedispersion to room temperature and then slowly adding isopropanol whilestirring the mixture. The charge oil was the same stock as described inExample I but had been treated with 10 lbs./bbl. of 99% sulfuric acid,neutralized by aqueous caustic soda and then dried by air blowing. Ithad a Doble test life of only 2 days.

The treatment was carried out by continuously passing the oil at atemperature of about 280 F. through the reactor at a rate providing aresidence time of minutes, continuously feeding in a stream of theisopropoxide slurry in amount such that the sodium content of thereaction mixture was 0.56% by weight and continuously passing CO intothe agitated mixture in molar excess of the isopropoxide.Sodium-containing material was separated from the reactor efiiuent bysedimentation and the treated oil was finished by contact with 20lbs./bbl. of adsorptive clay at 220 F. The treated cil product was foundto have a Doble test life of about 5 days, showing that it had goodstability for transformer service.

EXAMPLE VII Another continuous run was made in the same manner asdescribed above except that the sodium content of the reaction mixturewas increased to about 1.1% by weight. The Doble test life of the oilwas found to be about 7 days.

EXAMPLE VIII A sample of the acid-treated and neutralized charge oilused in Example VI was treated batchwise with tertiary butoxide as thetreating agent. The alkoxide was prepared by dispersing 0.5% by weightof sodium in the oil while hot, cooling to room temperature and slowlyadding 1.1 moles of tertiary butanol per mole of sodium. The mixture wasthen heated to about 280 F. and agitated for 30 minutes. Carbon dioxidewas then passed into the mixture at about 280 F. for 20 minutes.Sodium-containing material was removed from the mixture by settling. Thetreated oil had a Doble Test life of 5 days.

By way of comparison, when this run was repeated, except that n-propylalcohol in one case and ethyl alcohol in another case were substitutedfor tertiary butyl alcohol, the Doble test life was only 2 days whichwas the same value as obtained for the charge oil before treatment. Thisindicates that primary alcohols are not operative for the presentpurpose.

EXAMPLE IX The charge oil was another batch of naphthenic distillate oilhaving the following properties: A.P.*I.

6 gravity=24.8; flash point=290 F.; viscosity at 100 F.=58 S.U.S.;viscosity-gravity constant=0.88; R.I., d/ 20-=1.5000; totalaromatics=37.5% by weight; monocyclic aromatics=25.6% by weight;dicyclic aromatics =10.2% by weight; tricyclic aromatics=1.7% by weight;sulfur content=0.20% by weight; nitrogen content= p.p.m. The oil wastreated with 20 lbs./bbl. of 99% H SO then with 2 lbs./-bbl. ofcoagulation clay, neutralized with aqueous caustic soda, water washedand finally brightened by air blowing. At this stage the oil had a Dobletest life of only 2 days.

A series of runs was made in a batch-type pilot plant in which batchesof the acid-treated oil were treated with sodium isopropoxide formed insitu in the oil by the addition of metallic sodium and isopropanolthereto. The conditions in all the runs were substantially the same. Theamount of sodium used was 0.5% by weight on the oil, the molar ratio ofisopropanol to sodium was 1.1 and the temperature was 250-280 F.Following this treatment the oil was blown with CO at about the sametemperature level, the molar ratio of CO used to sodium being 1.2. Theoil was then agitated at about 175 F. with 0.4 volume water per volumeof oil, the mixture was settled for minutes and the water layer wasremoved. This washing procedure was repeated except that the temperaturewas about 145 F. The oil was then heated to about 225 C. while undervacuum and was sparged with nitrogen to remove residual water. Finallythe oil was contacted with 5 lbs./bbl. of clay for 30 minutes at 225 F.

In the foregoing manner the oils designated as C, D and E in FIGURES 2-Aand 2-B were obtained and their PFVO curves were found to be as showntherein. The Doble Test life values were found to be as follows:

Days Oil C 14 Oil D 10 Oil E 14 For these oils differences between thelife values found and between the curves shown in FIGURES 2-A and 2-Bmerely represent normal variations that can be expected for repetitiveprocessing operations and testing of products and are not considered tohave substantial significance with respect to quality of the oils.

The following are typical properties of oils C, D and E:

Comparison of these values with the properties given for the originalcharge stock shows that the most drastic change elfected by the treatingprocedure employed was in the nitrogen contents of the oils. A majorpart of the original nitrogen content (50 ppm.) was removed by the acidtreatment while a minor but highly significant part was removed by theNa isopropoxide-CO treatment. The total aromatic content was reducedonly slightly while the sulfur content was reduced to approximatelyone-half the original value. The latter changes occurred mainly duringthe acid treatment step.

We claim:

1. A transformer oil comprising a naphthenic petroleum distillateboiling in the range of 460-775 having a viscosity in the range of 50-65S.U.S. at F., a viscosity-gravity constant in the range of 0.84-0.92 anda nitrogen content less than 4 p.p.-m. and, when tested under -D0bleOxidation Test conditions in the absence of an added inhibitor,exhibiting the following characteristics: (1) a neutralization numberless than 0.25 mg. KOH/g. at 96 hours oxidation time; (2) absence ofsludge at 96 hours oxidation time; and (3) power factors during theoxidation period of 0-96 hours consistently less than 2 percent.

2. A transformer oil according to claim 1 having a nitrogen content notexceeding 2 ppm.

3. A transformed oil according to claim 1 having a power factor at 192hours of oxidation less than 2 percent.

4. A transformer oil according to claim 1 having a viscosity in therange of 55-60 S.U.S. at 100 F.

References Cited 5 UNITED STATES PATENTS 3,095,366 6/1963 Schiem'an208-14 1,856,700 5/1932 Ford 20814 3,000,807 9/1961 Wasson et a1. 208-4410 DELBERT E. GANTZ, Primary Examiner.

H. LEVINE, Assistant Examiner.

1. A TRANSFORMER OIL COMPRISING A NAPHTHENIC PETROLEUM DISTILLATEBOILING IN THE RANGE OF 460-775*F. HAVING A VISCOSITY IN THE RANGE OF50-65 S.U.S. AT 100*F., A VISCOSITY-GRAVITY CONSTANT IN THE RANGE OF0.84-0.92 AND A NITROGEN CONTENT LESS THAN 4 P.P.M. AND, WHEN TESTEDUNDER BOBLE OXIDATION TEST CONDITIONS IN THE ABSENCE OF AN ADDEDINHIBITOR, EXIBITING THE FOLLOWING CHARACTERISTICS: (1) A NEUTRALIZATIONNUMBER LESS THAN 0.2K MG. KOH/G. AT 96 HOURS OXIDATION TIME; (2) ABSENCEOF SLUDGE AT 96 HOURS OXIDATION TIME; AND (3) POWER FACTORS